1. Field of the Invention
The present invention relates to a thermal conductivity detector and a method for operating the same.
2. Description of the Related Art
Thermal conductivity detectors are used to detect certain liquid or gaseous substances (fluids) based on their characteristic thermal conductivity, particularly in gas chromatography. Here, components or substances of a gas mixture are separated by passing a sample of the gas mixture in a carrier gas (mobile phase) through a separation column containing a stationary phase. The different components interact with the stationary phase that causes each component to elute at a different time, known as the retention time of the component. The separated substances are detected by a thermal conductivity detector that includes has a measuring cell with an appropriate detector element, e.g., an electrically heated filament disposed in a measurement channel. Depending on the thermal conductivity of the substance flowing past the heated filament, more or less heat is diverted from the heating filament to the wall of the measurement channel, and the heating filament is correspondingly cooled to a greater or lesser degree. As a result of the cooling of the heating filament, its electrical resistance changes, which is detected.
For this purpose, the heating filament may be disposed in a measuring bridge, which contains additional resistors and an additional heating filament in a reference channel through which a reference fluid flows (e.g. U.S. Pat. No. 5,756,878, FIG. 8). The thermal conductivity of the substance passing the heating filament is obtained from an amount of energy which is supplied to the measuring bridge and is controlled to maintain the temperature of the heating filament at a predetermined temperature. Instead of the resistors, further filaments may be provided which are fluidically parallel or in series with the filaments in the measurement channel and the reference channel, respectively.
From FIG. 2 of US 2008/0291966 A1, e.g., a thermal conductivity detector is known that includes a measuring cell as well as a reference cell. The measuring cell is passed through by the carrier gas stream whereas the reference cell is passed solely by the carrier gas. The detector signals provided by the respective thermal conductivity detector elements of the measuring and reference cells are amplified and then subtracted from each other to obtain a difference signal that is only representative of the sample. For further digital processing, the difference signal must be digitized with a high resolution of, e.g., 24 bits which corresponds to a dynamic range of 144 dB. Thus, noise and drift characteristics of the difference signal are very important.
A problem with the additional reference cell is that it may have noise, drift and physical response characteristics that do not exactly match or track those of the measuring cell. These characteristics work against the quality of the detector signal from the measuring cell because they may introduce degradation in the difference signal when the reference signal is subtracted from the detector signal.
Additionally, chromatography applications can sometimes be limited by the number of reference cells available for measurement. Groups of detector cells are often assigned to one fixed reference cell. Thus, the ability to use any detector without a reference constraint would allow for far greater freedom to application chemist and would also reduce complexity and costs.